This invention relates to liquid cooled nuclear reactor constructions.
One known construction of liquid cooled nuclear reactor comprises a core supported on a diagrid within a steel primary vessel. The primary vessel depends from the roof of a prestressed concrete vessel which forms the primary containment. The core is submerged in a pool of liquid sodium and comprises a plurality of fuel element sub-assemblies closely packed side-by-side on the diagrid. The fuel element sub-assemblies each comprise a bundle of closely spaced slender fuel pins of a fissile material, the bundle being enclosed by a steel wrapper or shroud through which sodium coolant is flowed upwardly in heat exchange with the fuel pins. In the event of a loss of coolant flow, such as would be caused by a blockage of some of the fuel pin interspaces, the temperature of the fuel could rise sufficiently to cause it to melt and the resultant molten fuel could have sufficient heat capacity to cause it eventually to burn through the material of the primary vessel thereby releasing liquid sodium into the primary containment. One method of reducing such risk consists in providing a catchment tray for molten fuel debris below the diagrid. One such tray which has been proposed comprises a plurality of spaced elongate troughs of "V" cross-section arranged in layers, the troughs in any layer are parallel to each other but those of alternate layers are laid in directions normal to each other. The troughs of a third layer (which are laid in the same direction as those of the uppermost first layer) are off-set to the troughs of the first layer so as to provide catchment for debris falling through the spaces between the troughs of the first layer. Similarly the troughs of a fourth layer are off-set to those of the second layer. In this way a catchment tray of multiple layers may be built up to provide a barrier impervious to debris falling from the core but sufficiently pervious to enable liquid sodium to flow upwardly by natural or forced convection effectively to cool the troughs. However, difficulties are foreseen in providing an adequate system of support for the trays and an adequate restraint against thermal distortion that is effective during the time in which they contain molten fuel. Furthermore, if the trays were manufactured from exotic high melting point materials there could be problems in effecting satisfactory end closures.